Many electro-optical devices rely upon the controlled deflection of a light beam. The electrical control of optical beam deflection finds use in optical signal processing, displays, printing, scanning, optical drives, and other electro-optical devices.
The use of electro-optical deflection of a light beam has been recently extended to signal conversion. Nunnally et al, U.S. Pat. No. 6,714,149, discloses an analog signal conversion technique that is based upon the optical deflection of a light beam. In the invention of the '149 patent, an analog signal is converted to an optical deflection. Embodiments disclosed in the '149 patent include an N-bit parallel output. Embodiments disclosed in the '149 patent include a light beam deflected according to an analog signal being converted with a spatial filter into an N-bit binary light pattern that is then collected and sensed by optical detectors.
The invention of the '149 patent provided a new technique for the conversion of analog signals into digital signals, which is a fundamental task necessary in uncountable varieties of electronic devices. Conventional analog to digital converters (ADCs) had employed electronic circuitry and repetitive processing to define the digital value of an analog signal. Higher frequency signals require higher sampling rates, i.e., the rate at which an analog signal is measured for the purpose of determining a corresponding digital value. One guideline is known as the Nyquist criteria. The Nyquist criteria requires that the digital sampling rate be greater than two times the analog frequency in order to faithfully reproduce the analog signal from the digital values. In practice of the prior conventional devices, the sampling rate was typically chosen to be 5 times the analog frequency to be sampled. As frequencies of interest become high, e.g., in the tens of GigaHertz, conventional electronic techniques may not meet a desired rate of sampling. Even at lower frequencies, the electronics for a high quality analog to digital conversion may become complex and expensive. These problems were addressed by the invention of the '149 patent.
A limitation arises when analog signals venture into the microwave frequency range. Deflection may be achieved, as in preferred embodiments of the '149 patent, by passing a light beam through a birefringent crystal and deflecting the light beam by applying an electrical field to the crystal. The analog signal may be used to create the electric field and provide the basis for a deflection that can be converted into a digital value. At microwave frequency, however, the phase velocity of the microwave electric field and the optical electric field must be matched in an electro-optical crystal. At frequencies lower than microwave, the electric field may be considered constant during the passage of the optical wave through the crystal, but that does not hold true at microwave frequencies.
Electro-optical modulation has been achieved with microwave excitation through techniques that increase the speed of the electromagnetic voltage wave in a waveguide to match the phase velocity of the optical wave and the voltage wave. Waveguide phase matching is maintained over a relatively narrow frequency band due to the frequency dependence of the waveguide. Examples of these phase velocity matching techniques based upon increasing the phase velocity of the electro-magnetic wave in a waveguide are disclosed in the following publications: “Performance and Modeling of Broadband LiNBO3 Traveling waver Optical Intensity Modulator,” G. K. Golpalakrishnan et al, Journal of Lightwave Technology, Vol. 12, No. 10, October 1994; Ti:LiNBO3 Millimeter-Wave Optical Modulators,” K. Noguchi et al., Vol. 16, No. 4, April 1998.